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Fluorosurfactants for applications in catalysis
Accepted Manuscript Title: Fluorosurfactants for Applications in Catalysis Author: Quentin Jochyms Emmanuel Mignard Jean-Marc Vincent PII: DOI: Reference:
S0022-1139(15)00024-X http://dx.doi.org/doi:10.1016/j.jfluchem.2015.01.011 FLUOR 8497
To appear in:
FLUOR
Received date: Revised date: Accepted date:
16-12-2014 22-1-2015 24-1-2015
Please cite this article as: Q. Jochyms, E. Mignard, J.-M. Vincent, Fluorosurfactants for Applications in Catalysis, Journal of Fluorine Chemistry (2015), http://dx.doi.org/10.1016/j.jfluchem.2015.01.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Graphical Abstract - Pictogram
Fluorosurfactants as additives / catalysts
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in water, perfluorocarbons, scCO2, or organic solvents
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*Graphical Abstract - Synopsis
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The benefits and limitations to the use of fluorosurfactants in catalytic processes occurring in aqueous media or at the interface of two immiscible fluids (perfluorocarbons, organic solvents, scCO2) are described in a short review.
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*Highlights (for review)
Fluorosurfactants were employed in catalysis to favor reactions at interfaces Fluorosurfactants were often combined with perfluorocarbons or scCO2
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Fluorous catalysts have strong potential for application as surfactant catalysts
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*Manuscript
Fluorosurfactants for Applications in Catalysis
Quentin Jochyms,a,b Emmanuel Mignard,b,* Jean-Marc Vincenta,*
Univ. Bordeaux – CNRS UMR 5255, 351 Crs de la Libération, 33405 Talence (France)
b
CNRS, LOF, UMR 5258, F-33600 Pessac, France.
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a
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Fax : +33 540006158
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[email protected]
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Abstract Due to the specific physical and chemical properties of perfluorinated chains, i.e. stiffness, increased size and extreme hydrophobicity/lipophobicity, fluorosurfactants exhibit enhanced surfactant properties. Consequently, fluorosurfactants have found wide applications in
ip t
consumer products, industries and research laboratories. In this article, the use of fluorosurfactants for applications in catalytic processes is reviewed. The role and benefits of
cr
such additives in catalytic processes to favor the formation of micellar systems in water, in
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supercritical CO2 or in reactions taking place at liquid-liquid interfaces by favoring the formation of emulsions or by stabilizing microdroplets, will be discussed. Finally, an
an
interesting example of catalysis which takes place at the air-water interface thanks to a
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fluorous catalytic Langmuir film, will be highlighted.
Keywords: fluorosurfactant, catalysis, amphiphilic catalyst, micellar systems, emulsions,
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fluorous.
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Introduction To stabilize an emulsion made of a dispersion of one liquid in another, where the two liquids are immiscible, one must add specific molecules called surfactants, which is an abbreviation of SURFace-ACTive AgeNTS.[1] Surfactants exhibit two main properties: at
ip t
low concentrations these molecules show a strong tendency to absorb onto both air-liquid surfaces and liquid-liquid interfaces. In this way, surfactants reduce the interfacial tension (),
cr
i.e. the Gibbs free energy change G, and help to stabilize the droplets, i.e. the dispersed
us
phase in the continuous one. Moreover, immediately above the critical micelle concentration (CMC), i.e. when all surfaces and interfaces are saturated, the remaining surfactant starts to
an
aggregate and form micelles in the continuous phase. The simplest structure of surfactants consists in a molecule exhibiting two different moieties with a linker between the two: one
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side of the molecule is hydrophilic while the other is lipophilic (or hydrophobic). The hydrophilic or polar groups could be either ionic or nonionic structures, while the
ed
hydrophobic or apolar groups are usually constituted by one or several alkyl chains which
ce pt
could incorpor ate aryl groups. Thus, all surfactants have an amphiphilic structure. However, not all amphiphilic structures are capable to decrease the surface tension. Surfactants are widely used in the oil industry,[2] as well as for cosmetics,[3] or food-related applications.[4] However, it should be noted that the development and use of hydrocarbon
Ac
surfactants for applications in catalytic processes, in particular to allow reactions to be conducted in micellar aqueous solutions, is receiving considerable interest.[5] By this approach many reactions can be conducted very effectively without organic solvents, thus leading to a strong decrease of the environmental factor (E factor = amount of waste produced (kg)/amount of product (kg)).[6] At the same time, fluorosurfactants, i.e. surfactants modified with at least one perfluorinated chain, have been widely studied and employed in both industry and academic research
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laboratories, and have found numerous applications such as water/oil/stain repellents, additives in paints and coatings or to produce fluoropolymers using emulsion polymerization processes, or for the preparation of fluorinated colloids for biomedical applications.[7] Due to the specific properties of the perfluorinated chains, i.e. the stiffness, increased size and hydrophobicity/lipophobicity,
fluorosurfactants
exhibit
enhanced
surfactant
ip t
extreme
properties. They self-assemble into highly-ordered structures, lower the surface tension of
cr
water more effectively than their hydrogenated homologues and exhibit lower CMC values.
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Moreover, the lipophobic character of perfluoroalkyl tails, make them attractive to stabilize perfluorocarbons (PFCs)/hydrocarbons (HCs) interfaces and to generate micelles (reverse
an
micelles) into organic solvents, PFCs or supercritical CO2 (scCO2). In some cases, these particular chemicals can be called surfactant-like catalysts,[8] since concomitantly stabilize
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an interface and perform catalysis. In this short review we focus on the use of fluorosurfactants to facilitate catalytic processes occurring in aqueous media or at the
ce pt
such compounds.
ed
interface of two immiscible fluids (PFCs, HCs, scCO2), highlighting the benefits of using
1. Reactions under micellar conditions 1.1 Reaction in water
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Chiba and coworkers reported on the key role of micelle formation on the acceleration of Diels-Alder reactions.[9] They found that conducting the reaction with the LiFOS (lithium perfluorooctanesulfonate) fluorosurfactant and water was more efficient than when the reaction was conducted in an organic solvent. It appeared that the addition of perfluorohexane (PFH) in a stoechiometric amount with respect to LiFOS could further enhance the reaction rate (table 1), while using an excess of PFH led to a decrease of the rates. Particle size analysis has shown that a 1:1 ratio between LiFOS and PFH produces micellar solutions, whereas emulsions are obtained when PFH is used in excess. The authors suggested that the 5 Page 7 of 28
rate enhancement could be due to the large interfacial area resulting from the generation of micelles, the reactants concentrating in the interface domain between the repulsive fluorous and aqueous phases, thus favoring the intermolecular reaction.
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cr
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Table 1: Reaction rates for the reaction below and particle sizes for different compositions.[9]
-
100
an
11.6
0.365
42.4
100
200
165.2
19.3
100
500
M
LiFOS PFH Mean particle Reaction rate -1 -1 (mmol.L ) (mmol.L ) size (µm) (mmol.L-1.h-1)
225.9
11.5
200
200
N.D.
84.8
500
N.D.
142.5
-
100
ce pt
500
ed
100
Guo, Chen et al. prepared new fluorosurfactants (scheme 1) with the objective to avoid
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the use of acetonitrile, a co-solvent employed to improve the solubility of substrates for the radical addition of perfluoroalkyl iodides to alkenes (scheme 2) initiated by sodium dithionite conducted in water.[10] Only surfactant 1 (~ 5 mol%), which exhibits the best surfactant properties ( = 21 mN.m-1), led to an excellent yield (90%), as found when using the water/acetonitrile blend. With the less efficient surfactants 2-3 ( = 41 and 50 mN.m-1, respectively) the reaction proceeded with lower yields. It seems reasonable to ascribe the positive role of 1 to the dispersion of the fluorophilic/lipophilic substrates in water as a result
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of the formation of micelles. It should be noted that direct evidence for the formation of
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micelles was not provided.
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Scheme 1: Structural formulas of fluorous surfactants 1-3, and reaction studied.[10]
Considering hydrosoluble substrates/reagents, generating micellar aqueous solutions
ed
can be beneficial for reactions employing hydrophobic catalysts. Kondo and coworkers, exploited surfactants to favor the reactivity of copper catalysts towards reactants solubilized
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in water.[11] Copper salts were combined with two types of ligand (scheme 2) and various surfactants (F-PEG, Brij-30 or Triton X-100), and tested the coupling reaction between arylboronic acids and imidazoles. CuOAc2-4 (10 mol%) was combined with either Triton X-
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100 or Brij-30, while CuOAc-5 (10 mol%) was employed with F-PEG. They have shown that the yield of coupling product was approximately doubled when the surfactant is present (30 to 50 mol%). Exploiting the optimal conditions, a range of N-arylimidazoles was prepared in moderate to good yields, the choice of the best ligand/surfactant couple being substratedependent.
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Scheme 2: Ligands, surfactants and reaction studied in the work of Kondo and coworkers.[11]
In the previous examples, the surfactants used were not the catalytic species. Hanzawa
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et al. used the perfluooctanesulfonic acid (PFOSA) as a surfactant catalyst,[12] a so-called Brønsted-Acid-Surfactant-combined Catalyst.[13] Various acids were tested for the Pictet-
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Spengler reaction (scheme 3). For the model reaction, the best yield (90% after 18 h) was
ed
obtained with PFOSA employed at 20 mol% loading. For comparison, using the trifluoromethylsulfonic acid (20 mol%) the yield was 4%. The authors suggested that the
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substrates and the PFOSA formed aggregates in water. The addition of hexafluoroisopropanol was found to improve the reactivity, with a yield reaching 99% in only 4 h. Using the optimized conditions, they succeeded in preparing a range of tetrahydroisoquinolines in water
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with good to excellent yields.
Scheme 3: Pictet-Spengler reaction.[12]
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Similarly, Zhang et al. used the zinc perfluorooctanoate, Zn(PFO)2 as a Lewis Acid Surfactant Catalyst (LASC).[14] They characterized its surfactant properties by measuring its CMC (5 mM) and the minimum water/air surface tension (19.76 mN/m). It appeared that Zn(PFO)2 was a surfactant for water as well as for organic solvents. Zn(PFO)2 was applied as
ip t
catalyst (2.5 mol%) for the preparation of quinazolinones in water or water/ethanol mixtures. As shown in the figure 1, the solution micelle-containing solution turns from homogeneous
cr
(fig. 1, a) to an emulsion (fig. 1, b) as soon as the non-soluble reactants were introduced. At
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the end of the reaction, the products which precipitate upon cooling the reaction mixture were recovered by filtration, while the filtrate containing the micellar catalyst (fig.1, c) could be
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ed
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an
reused for subsequent reactions.
Figure 1: Photographs of: (a) Zn(PFO)2 in water; (b) emulsion solution after adding reagents;
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(c) the precipitated product at the end of the reaction (extracted from [14]).
1.2 In scCO2
The direct incorporation of fluorinated groups to ligands allows consideration of the
use of their metal complexes as surfactant catalysts at the interface of dispersed and continuous phases. When one phase consists of scCO2, it entails additional advantages: Firstly, scCO2 allow the replacement of harmful and expensive organic solvents, conducive with improved sustainability in chemical processes. Secondly, chemists have additional means to achieve more effective syntheses since the solvation properties of scCO2 can be 9 Page 11 of 28
finely tuned by adjusting the pressure and temperature. The catalytic hydrocarboxylation of terminal alkenes with CO and a surfactant-like catalyst were conducted at high pressure in inverse systems, i.e. liquid water-in-supercritical CO2 (scheme 4).[15] The catalyst is formed in situ from precatalyst [PdCl2(NCPh)2] and phosphine ligands. The latter could contain one
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or several short fluorinated groups (-CF3).
system
was
the mixture of 1 mM
of palladium
precatalyst
with
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The best
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Scheme 4: Pd-catalyzed hydrocarboxylation of terminal alkenes in supercritical conditions.[15]
tris(4-trifluoromethylphenyl)phosphine, oxalic acid, 1-octene and water in a molar ratio
ed
1:4:62.5:62.5:500, respectively, with the following optimal experimental conditions: 12 h at 90 °C and 3 MPa of CO for a total pressure of 15 MPa after CO2 addition. Interestingly, high
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conversion (55 %) and selectivity (90 %) were achieved when the precatalytic system was not soluble in the supercritical mixture of CO and CO2. At higher pressure, when the precatalyst becomes soluble in the supercritical medium, almost no conversion was observed. The best
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results were obtained when a perfluorinated surfactant (Krytox®) was added to the system (93 % conversion and 77 % selectivity). Decreasing the amount of the surfactant leads to a lower conversion while selectivity is not affected. Although no physico-chemical study of the catalytic system was conducted, the improvement of the conversion could be ascribed to the ambivalent character of the surfactant catalyst and the addition of the perfluorinated cosurfactant. Indeed, the latter was used in order to improve the generation and stabilization of smaller water droplets into the supercritical medium, and thus to increase the interfacial area, i.e. where the catalyst could be confined thanks to the fluorinated moieties of its ligands. 10 Page 12 of 28
Moreover, one can note that venting the reactor containing the supercritical medium allowed extraction of the carboxylic acids produced in a cold trap. Such water/scCO2 medium was also used with success by Tsang et al. to perform the catalytic air oxidation of toluene.[16] The authors synthesized the surfactant catalyst
ip t
[CF3(CF2)8COO]2Co·nH2O and used it with NaBr promoter in a water-in-scCO2 system containing toluene and O2 (1 MPa) at a total pressure of 15 MPa and 120 °C (scheme 5). After
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an induction period corresponding to the wetting of the scCO2, their catalyst showed excellent
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activity in these conditions: 98 % conversion, 99 % selectivity to benzoic acid and a high turnover frequency (TOF = 6.19 x 10-3 s-1), while poor activity was observed if NaBr or water
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was not included in the chemical system. Interestingly, their unoptimized catalyst is about one or two orders of magnitude more reactive than regular catalytic systems run in acetic acid-
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water or employing solid Co in scCO2. They attributed such high activity to the intrinsically faster transfers occurring in supercritical fluids than in conventional liquids, the dynamic
ed
properties of the emulsion, and the local high concentration of chemicals in the emulsion droplets and at the interface of the disperse and continuous phases. However, they did not
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show whether this was a truly stable microemulsion or a coarsely dispersed biphasic system. It is interesting to note that at standard ambient temperature and pressure, i.e. 25 °C and 0.1 MPa, the catalytic system is a solid, and hence a solid foam was obtained after rapid
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depressurization of the reactor. The authors also demonstrated that after a first complete air oxidation of toluene, the inverse catalytic system containing the surfactant-like catalyst can be reused for a second run with no sign of deactivation.[17] Moreover, the authors showed that by shortening the fluorinated tail of the anionic surfactant CF3(CF2)nCOO- a significant increase of the reaction rates was observed (reaction times of 9.7, 10.5 and 12.3 h for n = 3, 8 and 12, respectively). As a small change in the surfactant structure can modify the surface properties of the micelle, it significantly alters the reactivity of the substrates. In other words,
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the toluene which was confined in the supercritical phase could reach the catalytic head of the surfactant-like catalyst more easily when it bore a short fluorinated tail. Again, this work shows the advantage of designing and using a surfactant catalyst when reactants of different polarities are located in different solvents. Use of scCO2 enhances the heat and mass transfer
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as well as improving the process by allowing an easier separation and reusability of the
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catalytic micellar system.
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Scheme 5. Schematic representation of the catalytic system in a water-in-scCO2 mixture (extracted from [17]).
The same group also reported the use of a surfactant catalyst based on the combination
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of ammonium perfluorotetradecanoate and palladium nitrate (1:1 molar), applied to the hydrogenation of citral under wet scCO2 conditions (scheme 6).[18] A slight excess of water was added to the wet CO2 in order to obtain a single and transparent supercritical phase at 40 °C and 14 MPa containing micelles hosting Pd nanoparticles (NPs). The excess of water-tosurfactant molar ratio must be between 2 to 30 to obtain monodispersed Pd NPs of ~4 nm with a cubo-octahedral shape. To achieve the reaction, the reactor was then filled with 1 MPa of H2 (the total pressure was then increased up to 15 MPa) and the hydrogenation of citral was
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conducted during 1 h until completion. Surprisingly, an unusual selectivity was obtained in this inverse liquid water-scCO2 microemulsion, the citronellal being the preferred product (68 %) compared to dihydrocitronellal (12 %) respectively, as opposed to vapor (1 % and 82 %) or solution processes (19 % and 66 %). This result was explained by the authors by the size
ip t
and the shape of the micelles that consists in a water core containing the Pd nanoparticles surrounded by the extended long fluorinated chains of the surfactant. Hence, citral could align
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itself to allow its hydrophilic head to enter in the water core of the micelles. This behavior
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should allow the selective hydrogenation of the conjugated double bond that should be
ed
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an
near the Pd surface.
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Scheme 6. Products formed upon hydrogenation of citral.
2. Reactions under biphasic conditions 2.1 In emulsions
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Generating an emulsion leads to an increase of the surface area between two immiscible phases. If the emulsion is stabilized by a fluorosurfactant catalyst, reactions could then be potentially conducted effectively under biphasic conditions, while keeping the reactants/substrates and the catalyst compartmentalized, thus facilitating the product isolation and catalyst recycling by simple decantation. Since the publication by Horváth and Rábai in 1994 of a seminal paper on the Fluorous Biphasic Catalysis (FBC) [19], the most commonly employed metal ligands have been modified with perfluoroalkyl tags, and their metal complexes prepared and characterized to be employed in catalytic processes [20]. Depending 13 Page 15 of 28
on the fluorophilicity of the complexes, convenient and very efficient catalysts recovery procedures were developed based on liquid/liquid or solid/liquid separation protocols. When considering the chemical structure of the fluorous complexes of transition metals, it appears that they are intrinsically amphiphilic compounds. They typically possess a hydrophobic or
ip t
even lipophobic domain due to the presence of at least one perfluoralkyl chain (typically –C8F17) in their structure, while the hydrophilic domain should be provided by the metal
cr
ion/counterion couple. Most of the fluorous metal complexes described up to now could
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potentially behave as moderate to very strong surfactants, with the possibility of stabilizing, depending on their fluorophilicity/lipophilicity, not only HC/water interfaces, but also
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PFC/water or PFC/HC interfaces. It is also expected that the rigidity of their perfluoroalkyl tags will favor their self-organization at interfaces into highly organized and densely-packed
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structures. Surprisingly, to the best of our knowledge, despite the great number of fluorous transition-metal complexes developed so far and employed in multiphasic catalytic systems,
ed
no studies have been reported yet to try to evaluate their surfactant character, in particular by assessing their ability to lower surface tensions of liquid/liquid interfaces. Nonetheless, we
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wish to highlight below some examples of fluorous biphasic catalysis, in which catalysis occurs at liquid/liquid or air/liquid interfaces and for which the surfactant properties of the fluorous catalyst may be invoked to account for the observed reactivity.
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Reiser et al. prepared the fluorous “click” TEMPO derivative 6 (scheme 7) and
showed that this compound promotes the formation of an emulsion in a DCM/water mixture [21]. It was found that its solubility was very low in most solvents including DCM, perfluoromethylcyclohexane (≤ 1 mg/10 mL) and water. Under heterogeneous DCM/water conditions and using NaOCl as oxidant, 6 (0.2 to 1 mol%) proved a highly reactive and selective catalyst for the oxidation of alcohols to aldehydes. Its reactivity was even slightly higher than that observed with the pristine TEMPO which was fully soluble in the reaction
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conditions and did not emulsify the biphasic reaction mixture. The authors suggested that the high reactivity of 6 could be attributed to the emulsification of the reaction mixture by the catalyst, leading to a surfactant effect. Most interestingly, the recycling of 6 could be achieved by simple filtration followed by washings, while no loss of activity was noticed in four runs
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cr
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using an initial amount of 1 mol% of 6.
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6
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Scheme 7: Structure of the fluorous TEMPO 6 (Rf8 = C8F17) and photograph of the emulsified biphasic reaction mixture containing 6 (0.2 mol%) and the substrate/reactants (photograph
ed
extracted from [21]).
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Nishikido and coworkers reported on a practical fluorous biphasic process for small and bench-scale esterifications catalyzed by the lanthanide(III) complex Yb[N(SO2-nC8F17)2]3.[22] They developed a continuous-flow apparatus consisting of a reactor with a
Ac
mechanical stirrer and a separate decanter where phase-separation occurs (scheme 8). Reactions were conducted at 40 °C under biphasic conditions, the fluorous catalyst being immobilized in a PFC, while the substrate (alcohol)/reactant (acetic anhydride) are dissolved in toluene. The authors highlighted the fact that the reaction were conducted under vigorous stirring and proceeded in the resultant emulsion. Using a semi-industrial set up (reactor of 500 mL) the acetylation of cyclohexanol by acetic anhydride proceeded successfully for more than 500 h with high TON (~10000) attained for the catalyst whilst very low leaching of Yb was detected. This catalytic system also proved to be highly efficient for the Baeyer-Villiger 15 Page 17 of 28
reaction. Overall, this work clearly demonstrated the potential of such continuousflow/fluorous biphase systems for industrial applications.
during reaction
products in organic solvent
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before reaction
emulsion
organic
substrates in organic solvent
fluorous
an
reactor
Phase separation
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decanter
cr
catalyst immobilised in fluorous phase
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Scheme 8: Continuous-flow reaction model based on FBC (adapted from [22]).
Other interesting examples of catalytic reactions conducted using emulsified PFC/HC
ed
biphasic systems were reported by Gladysz and co-workers [23] and by Friesen, Montgomery and co-workers[24]. They prepared fluorous Wilkinson-type catalysts 7 [23] and 8,[24] the
ce pt
main difference being that 8 incorporated perfluoropolyalkylether chains instead of typical – C8F17 ponytails (scheme 9). Perfluoropolyalkylether chains are of particular interest as they raise less environmental concerns than the –CnF2n+1 chains and provide increased
Ac
fluorophilicity.[25] The hydrogenation of 2-cyclohexen-1-one catalysed by 7 [23] or 8 [24] was tested in the conditions of initially employed by Gladysz and co-workers,[24] i.e. under 1 atm of H2 at 45 °C in a biphasic perfluoromethylcyclohexane (PFMCH):toluene mixture. The reactions were conducted under vigorous stirring, the term emulsion being employed by Friesen and Montgomery to describe the reaction mixture. Both catalysts exhibited excellent activities with TOF values of 31 h-1 and 47 h-1 for 8 and 7, respectively. Interestingly, it was noticed that the reactions proceeded more effectively under the “emulsified biphasic conditions” than under homogeneous monophasic conditions,[23] for instance employing a 16 Page 18 of 28
1:3:3 solution of toluene:hexanes:PFMCH which forms a single phase at 45 °C. Whether this increased reactivity could be attributed to a surfactant effect brought or amplified by the fluorous catalyst is possible, but further studies would be necessary to assess this interesting
8
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7
cr
ip t
point.
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Scheme 9: Structures of the fluorous Wilkinson’s-type catalysts 7 [23] and 8.[24]
Yi and Cai conducted organic reactions at a water-fluorous solvent interface. They
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showed that the synthesis of benzodiazepines (scheme 10), through the condensation of ophenylenediamine and various ketones, could be conducted very effectively at room
ed
temperature when running the reactions in an aqueous/fluorous emulsion, the best results
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being obtained for the water/perfluorooctane/KFOS (potassium perfluorooctanesulfonate) system.[26] When using water, an organic solvent, or a PFC only as reaction medium, the reaction did not proceed, while in a water/PFC mixture without surfactant the reaction yield was very low. They proposed that the rate enhancement was due to the confinement of the
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reactants at the water/PFC interface generated in the stable emulsion which formed upon stirring, the average particle size diameter being ~ 3 microns.
Scheme 10. Synthesis of benzodiazepines.[26]
17 Page 19 of 28
2.2 In microfluidic devices During the last decade, droplet-based milli/microfluidic devices have received considerable interest.[27] As far as chemical reactions are concerned, reactions are typically conducted within individual microdroplets in an immiscible continuous phase which carry
ip t
these individual microreactors in microfluidic channels. PFCs have emerged as particularly appealing fluids for the continuous phase as they can carry either water-based or organic
cr
solvent-based microdroplets. In 2009, Theberge, Huck and co-workers exploited a fluorous
us
Pd(II) complex as a PFC-soluble catalyst with amphiphilic properties which could accumulate at the PFC-water interface and thus favor the reactivity at the interface between the reactants
an
solubilised in the water droplets and the catalyst dissolved in the continuous fluorous phase (scheme 11).[28] Using the setup presented in scheme 11, Suzuki couplings have been
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conducted from a range aryl bromides and aryl boronic acids affording the corresponding biaryls in good to excellent yields using residence times ranging from 0.75 to 8 h. Continuous
ed
recycling of the catalyst was successfully achieved by recirculating the fluorous phase, the catalyst being still active after having being recirculated approximately four times. The
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catalyst leaching from the fluorous phase was found very low, i.e. less than 2% of the total palladium. Interestingly, despite the precipitation of most of the biaryls forming within the
Ac
aqueous droplets, the plugging of the PTFE tube was not observed.
18 Page 20 of 28
a d
c
e
cr
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b
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Scheme 11: a) Structure of the fluorous Pd catalyst; b) schematic representation of a water droplet carried in a channel by the fluorous phase containing the catalyst; c) Schematic
an
representation of the microfluidic set-up; d) T-junction for the droplet formation; e)
3. Reaction at the air-water interface
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Separating column. Adapted from [28].
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An important contribution was made by Mingotaud and co-workers who exploited Langmuir films as a tool to study how the orientation/confinement of a catalyst, as found in
ce pt
heterogeneous or densely packed catalytic systems, may influence its reactivity.[29] Indeed, Langmuir films represent a unique opportunity to finely control and tune the density and orientation of a catalyst at the air-water interface, and thus study the influence of such
Ac
parameters on the catalytic activity. They chose the fluorous Mn-salen complex 9 as the amphiphilic catalyst for the epoxidation of the cinnamyl alcohol using the Urea/Hydrogen Peroxide (UHP) as the oxygen source (scheme 12).
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ip t
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Scheme 12: Structure of the amphiphilic catalyst 9 and reaction studied by Mingotaud and co-
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workers.[29)]
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Complex 9 forms a homogeneous and stable Langmuir film at the air/water interface, the molecular area at collapse being found at around 115 Å2. Interestingly, when the UHP was
M
present in the aqueous phase, the monolayer was found to be denser at a given pressure, thus suggesting a strong interaction with 9. Catalytic experiments were conducted by spreading 9
ed
on the aqueous phase containing the alcohol and UHP and determining the % of epoxide
ce pt
formed after a given time for different compression states of the film (figure 2). These data show that when compressing the film until a molecular area of ~ 150 Å2 (surface pressure kept at 0), a linear-dependency of the apparent rate with the molecular density is observed, as expected as no molecular organisation is present in the liquid phase of the monolayer. This
Ac
also agrees with observations in bulk solutions for which the reaction was found to be firstorder with respect to the catalyst. When compressing the film further (area < 150 Å2), a strong enhancement of the epoxide formation was observed, the maximum being attained for an area per molecule of about 140 Å2. It is proposed that the increased reactivity is due to the modification of the catalyst orientation during compression, PM-IRRAS measurements revealing an increase in the monolayer thickness during compression, as well as different orientations of 9 at low and high surface pressures. At very high surface pressures (molecular
20 Page 22 of 28
area below 135 Å2), a sharp decrease of the recovered epoxide was observed, most probably explained by the slow collapse of the film. By exploiting the “natural” surfactant properties of a fluorous complex and its ability to self-assemble into a highly organised langmuir film at the air/water interface it is thus possible to get important information on the influence of the
ip t
organisation of a catalyst in a surface-confined environment. There is little doubt that this approach exploiting fluorous catalysts could be extended to the study of enantioselective
cr
catalytic processes, or to systems which form Langmuir films at the air/organic solvent
ce pt
ed
M
an
us
interface.
Figure 2: Molar percentage of epoxide recovered from the subphase after 6 h of reaction for
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various areas per molecule of 9 (extracted from [29b]).
Conclusion
While fluorosurfactants are used in a broad range of applications, their use as additives in catalytic processes has received little attention. The examples highlighted in the review show the potential of fluorosurfactants in favoring reactions that take place at the interface between two phases, in particular if one of the phases is a PFC or scCO2. We expect that
21 Page 23 of 28
studies in this field will be pursued. During the last 20 years, with the development of the fluorous chemistry, almost all catalysts (metal-based or organocatalysts) have been developed in a fluorous version. Such compounds, which integrate at least one perfluoroalkyl chain in their structure should exhibit significant surfactant properties. As shown by the works of
ip t
Reiser or Theberge, such fluorous catalysts have a great potential for reactions conducted in liquid/liquid processes, by stabilizing emulsions and/or accumulating at the interface of
cr
PFC/water or PFC/HC biphasic systems. However, more studies need to be conducted to
us
assess the surfactant properties of the fluorous catalysts, and thereby allowing evaluation of the extent to which the surfactant character of the catalyst is crucial for the observed activity.
an
When conducting reactions under homogeneous micellar conditions with conventional surfactants, the separation of the surfactant from the products could be problematic. If using
M
fluorosurfactants, Fluorous Solid-Phase Extraction (FSPE) using fluorous silica-gel as the stationary phase, may represent a powerful separation technique to recover/recycle the
ed
surfactant.[30] Finally, the environmental concerns raised by the accumulation of the perfluorinated compounds in the environment and, for some of them, in living organisms
ce pt
(PFOA, PFOS…), could represent a strong limitation for further developments in this area.[31] However, one should note that for applications in catalytic processes, the confinement of the reactants coupled to effective recovery techniques, will obviously lead to
Ac
minimal leaching into the environment. We thus believe that environmental issues should not hamper innovative researches for applications in catalysis. However, further studies in the field should favor the use of fluorosurfactants with less environmental/health impact, for instance those bearing perfluoroalkylether or shorter perfluoroalkylated (≤ -C6F13) fragments.
22 Page 24 of 28
Acknowledgments The authors warmly thank the CNRS and Solvay for funding this work. The University of Bordeaux, the Région Aquitaine as well as Solvay RIC Bordeaux are also acknowledged for
ip t
their support.
a) M.J. Rosen, J.T. Kunjappu, in: Surfactants and Interfacial phenomena, 4th Ed., John
us
[1]
cr
References.
Wiley & Sons, Inc., 2004.
[2]
an
b) D. Myers, in: Surfactants Science and Technology, 3rd Ed., Wiley-Blackwell, 2005. L.L. Schramm (Eds), Surfactants: Fundamentals and Applications in the Petroleum
M. Rieger, L.D. Rhein (Eds), Surfactants in Cosmetics (Surfactant Science), CRC Press (2nd Ed), 1997.
[4]
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M
Industry, Cambridge University Press; Reissue edition, 2010.
G.L. Hasenhuettl, R.W. Hartel (Eds), Food Emulsifiers and Their Applications,
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Springer-Verlag New York Inc. (2nd Ed), 2008. a) T. Dwars, E. Paetzold, G. Oehme, Angew. Chem. Int. Ed. 44 (2005) 7174-7199. b) B.H. Lipshutz, S. Ghorai, Green Chem. 16 (2014) 3660-3679. a) R.A. Sheldon, Green Chem. 9 (2007) 1273-1283.
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[6]
b) R.A. Sheldon, Chem. Commun. (2008) 3352-3365.
[7]
a) E. Kissa, Fluorinated Surfactants and Repellents: Second Edition, Revised and Expanded, Surfactant Science Series, Vol. 97, Marcel Dekker: New York, 2001. b) M.-P. Krafft Adv. Drug. Deliv. Rev. 47 (2001) 209-228. c) M.-P. Krafft, J.G. Riess Chem. Rev. 109 (2009) 1714-1792.
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B. Zhang, Z. Jiang, X. Zhou, S. Lu, J. Li, Y. Liu, C. Li, Angew. Chem., Int. Ed. 51 (2012) 13159-13162.
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a) K. Nishimoto, Y. Okada, S. Kim, K. Chiba, Electrochimica Acta 56 (2011) 1062610631.
ip t
b) K. Nishimoto, S. Kim, Y. Kitano, M. Tada, K. Chiba, Org. Lett. 8 (2006) 55455547.
Z. Xiao, H. Hu, J. Ma, Q. Chen, Y. Guo, Chin. J. Chem. 31 (2013) 939-944.
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K. Inamoto, K. Nozawa, J. Kadokawa, Y. Kondo, Tetrahedron 68 (2012) 7794-7798.
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A. Saito, J. Numagachi, Y. Hanzawa, Tetrahedron Lett. 48 (2007) 835-839.
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a) S. Kobayashi, K. Manabe, in Interfacial Nanochemistry, Springer (2005).
an
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b) M. Shiri, M. Ali Zolfigol, Tetrahedron 65 (2009) 587-598. L.-M. Wang, L. Hu, J.-H. Shao, J. Yu, L. Zhang, J. Fluorine Chem. 129 (2008) 1139-
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C.T. Estorach, A. Orejón, N. Ruiz, A.M. Masdeu-Bultó, G. Laurenczy, Eur. Inorg.
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[15]
Chem. (2008) 3524-3531.
J. Zhu, A. Robertson, S.C Tsang, Chem. Commun. (2002) 2044-2045.
[17]
J. Zhu, S.C. Tsang, Catalysis Today, 81 (2003) 673-679.
[18]
P. Meric, K.M.K. Yu, S.C. Tsang, Langmuir 20 (2004) 8537-8545.
[19]
I.T. Horváth, J. Rábai, Science 266 (1994) 72-75.
Ac
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[16]
a) G. Pozzi, I. Shepperson, Coord. Chem. Rev. 242 (2003) 115-124. b) J.-M. Vincent, Top. Curr. Chem. 308 (2012) 153-174.
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A. Gheorghe, T. Chinnusamy, E. Cuevas-Yañez, P. Hilgers, O. Reiser, Org. Lett. 10 (2008) 4171-4174.
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a) A. Yoshida, X. Hao, J. Nishikido, Green Chem. 5 (2003) 554-557.
24 Page 26 of 28
b) X. Hao, O. Yamazaki, A. Yoshida, J. Nishikido Tetrahedron Lett. 44 (2003) 49774980. [23]
T. Soós,
B.L.
Bennett, D. Rutherford,
L.P. Barthel-Rosa, J.A. Gladysz
Organometallics 20 (2001) 3079-3086. C. M. Friesen, C.D. Montgomery, S.A.J.U. Temple J. Fluorine Chem. 144 (2012) 24-
ip t
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a) M. Skalicky, V. Skalická, J. Paterová, M. Rybáčková, M. Kvíčalová, J. Cvačka, A.
cr
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Březinová, J. Kvíčala, Organometallics 31 (2012) 1524-1532.
b) R. Corrêa da Costa, T. Buffeteau, A. Del Guerzo, N.D. McClenaghan, J.-M.
an
Vincent Chem. Commun. 47 (2011) 8250-8252.
c) G. Pozzi, M. Cavazzini, S. Quici, S. Fontana Tetrahedron Lett. 38 (1997) 7605-
M
7608.
W.-B. Yi, C. Cai, J. Fluor. Chem. 130 (2009) 1054-1058.
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A. Günther, K.F. Jensen, Lab Chip 6 (2006) 1487-1503.
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A.B. Theberge, G. Whyte, M. Frenzel, L.M. Fidalgo, R.C.R. Wootton, W.T.S. Huck
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[29]
ce pt
Chem. Commun. (2009) 6225-6227.
a) A. Pasc-Banu, C. Sugisaki, T. Gharsa, J.-D. Marty, I. Gascon, G. Pozzi, S. Quici, I. Rico-Lattes, C. Mingotaud Angew. Chem. Int. Ed. 43 (2004) 6174-6177.
Ac
b) A. Pasc-Banu, C. Sugisaki, T. Gharsa, J.-D. Marty, I. Gascon, Michael Krämer, G. Pozzi, B. Desbat, S. Quici, I. Rico-Lattes, C. Mingotaud Chem. Eur. J. 11 (2005) 6032-6039. [30]
a) M.-P. Krafft, F. Jeanneaux, M. Le Blanc, J.G. Riess, A. Berthod Anal. Chem. 60 (1988) 1969-1972. b) D.P. Curran Synlett (2001) 1488-1496.
25 Page 27 of 28
a) A.B Lindstrom, M.J. Strynar, E.L. Libelo Environ. Sci. Technol.45 (2011) 79547961. b) Z. Wang, I.T. Cousins, M. Scheringer, R.C. Buck, K. Hungerbühler Environ. Int. 69
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M
an
us
cr
ip t
(2014) 166-176.
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[31]
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S0022-1139(15)00024-X http://dx.doi.org/doi:10.1016/j.jfluchem.2015.01.011 FLUOR 8497
To appear in:
FLUOR
Received date: Revised date: Accepted date:
16-12-2014 22-1-2015 24-1-2015
Please cite this article as: Q. Jochyms, E. Mignard, J.-M. Vincent, Fluorosurfactants for Applications in Catalysis, Journal of Fluorine Chemistry (2015), http://dx.doi.org/10.1016/j.jfluchem.2015.01.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Graphical Abstract - Pictogram
Fluorosurfactants as additives / catalysts
B
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in water, perfluorocarbons, scCO2, or organic solvents
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A
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*Graphical Abstract - Synopsis
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The benefits and limitations to the use of fluorosurfactants in catalytic processes occurring in aqueous media or at the interface of two immiscible fluids (perfluorocarbons, organic solvents, scCO2) are described in a short review.
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*Highlights (for review)
Fluorosurfactants were employed in catalysis to favor reactions at interfaces Fluorosurfactants were often combined with perfluorocarbons or scCO2
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Fluorous catalysts have strong potential for application as surfactant catalysts
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*Manuscript
Fluorosurfactants for Applications in Catalysis
Quentin Jochyms,a,b Emmanuel Mignard,b,* Jean-Marc Vincenta,*
Univ. Bordeaux – CNRS UMR 5255, 351 Crs de la Libération, 33405 Talence (France)
b
CNRS, LOF, UMR 5258, F-33600 Pessac, France.
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a
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Fax : +33 540006158
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[email protected]
1 Page 4 of 28
Abstract Due to the specific physical and chemical properties of perfluorinated chains, i.e. stiffness, increased size and extreme hydrophobicity/lipophobicity, fluorosurfactants exhibit enhanced surfactant properties. Consequently, fluorosurfactants have found wide applications in
ip t
consumer products, industries and research laboratories. In this article, the use of fluorosurfactants for applications in catalytic processes is reviewed. The role and benefits of
cr
such additives in catalytic processes to favor the formation of micellar systems in water, in
us
supercritical CO2 or in reactions taking place at liquid-liquid interfaces by favoring the formation of emulsions or by stabilizing microdroplets, will be discussed. Finally, an
an
interesting example of catalysis which takes place at the air-water interface thanks to a
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fluorous catalytic Langmuir film, will be highlighted.
Keywords: fluorosurfactant, catalysis, amphiphilic catalyst, micellar systems, emulsions,
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fluorous.
2 Page 5 of 28
Introduction To stabilize an emulsion made of a dispersion of one liquid in another, where the two liquids are immiscible, one must add specific molecules called surfactants, which is an abbreviation of SURFace-ACTive AgeNTS.[1] Surfactants exhibit two main properties: at
ip t
low concentrations these molecules show a strong tendency to absorb onto both air-liquid surfaces and liquid-liquid interfaces. In this way, surfactants reduce the interfacial tension (),
cr
i.e. the Gibbs free energy change G, and help to stabilize the droplets, i.e. the dispersed
us
phase in the continuous one. Moreover, immediately above the critical micelle concentration (CMC), i.e. when all surfaces and interfaces are saturated, the remaining surfactant starts to
an
aggregate and form micelles in the continuous phase. The simplest structure of surfactants consists in a molecule exhibiting two different moieties with a linker between the two: one
M
side of the molecule is hydrophilic while the other is lipophilic (or hydrophobic). The hydrophilic or polar groups could be either ionic or nonionic structures, while the
ed
hydrophobic or apolar groups are usually constituted by one or several alkyl chains which
ce pt
could incorpor ate aryl groups. Thus, all surfactants have an amphiphilic structure. However, not all amphiphilic structures are capable to decrease the surface tension. Surfactants are widely used in the oil industry,[2] as well as for cosmetics,[3] or food-related applications.[4] However, it should be noted that the development and use of hydrocarbon
Ac
surfactants for applications in catalytic processes, in particular to allow reactions to be conducted in micellar aqueous solutions, is receiving considerable interest.[5] By this approach many reactions can be conducted very effectively without organic solvents, thus leading to a strong decrease of the environmental factor (E factor = amount of waste produced (kg)/amount of product (kg)).[6] At the same time, fluorosurfactants, i.e. surfactants modified with at least one perfluorinated chain, have been widely studied and employed in both industry and academic research
4 Page 6 of 28
laboratories, and have found numerous applications such as water/oil/stain repellents, additives in paints and coatings or to produce fluoropolymers using emulsion polymerization processes, or for the preparation of fluorinated colloids for biomedical applications.[7] Due to the specific properties of the perfluorinated chains, i.e. the stiffness, increased size and hydrophobicity/lipophobicity,
fluorosurfactants
exhibit
enhanced
surfactant
ip t
extreme
properties. They self-assemble into highly-ordered structures, lower the surface tension of
cr
water more effectively than their hydrogenated homologues and exhibit lower CMC values.
us
Moreover, the lipophobic character of perfluoroalkyl tails, make them attractive to stabilize perfluorocarbons (PFCs)/hydrocarbons (HCs) interfaces and to generate micelles (reverse
an
micelles) into organic solvents, PFCs or supercritical CO2 (scCO2). In some cases, these particular chemicals can be called surfactant-like catalysts,[8] since concomitantly stabilize
M
an interface and perform catalysis. In this short review we focus on the use of fluorosurfactants to facilitate catalytic processes occurring in aqueous media or at the
ce pt
such compounds.
ed
interface of two immiscible fluids (PFCs, HCs, scCO2), highlighting the benefits of using
1. Reactions under micellar conditions 1.1 Reaction in water
Ac
Chiba and coworkers reported on the key role of micelle formation on the acceleration of Diels-Alder reactions.[9] They found that conducting the reaction with the LiFOS (lithium perfluorooctanesulfonate) fluorosurfactant and water was more efficient than when the reaction was conducted in an organic solvent. It appeared that the addition of perfluorohexane (PFH) in a stoechiometric amount with respect to LiFOS could further enhance the reaction rate (table 1), while using an excess of PFH led to a decrease of the rates. Particle size analysis has shown that a 1:1 ratio between LiFOS and PFH produces micellar solutions, whereas emulsions are obtained when PFH is used in excess. The authors suggested that the 5 Page 7 of 28
rate enhancement could be due to the large interfacial area resulting from the generation of micelles, the reactants concentrating in the interface domain between the repulsive fluorous and aqueous phases, thus favoring the intermolecular reaction.
us
cr
ip t
Table 1: Reaction rates for the reaction below and particle sizes for different compositions.[9]
-
100
an
11.6
0.365
42.4
100
200
165.2
19.3
100
500
M
LiFOS PFH Mean particle Reaction rate -1 -1 (mmol.L ) (mmol.L ) size (µm) (mmol.L-1.h-1)
225.9
11.5
200
200
N.D.
84.8
500
N.D.
142.5
-
100
ce pt
500
ed
100
Guo, Chen et al. prepared new fluorosurfactants (scheme 1) with the objective to avoid
Ac
the use of acetonitrile, a co-solvent employed to improve the solubility of substrates for the radical addition of perfluoroalkyl iodides to alkenes (scheme 2) initiated by sodium dithionite conducted in water.[10] Only surfactant 1 (~ 5 mol%), which exhibits the best surfactant properties ( = 21 mN.m-1), led to an excellent yield (90%), as found when using the water/acetonitrile blend. With the less efficient surfactants 2-3 ( = 41 and 50 mN.m-1, respectively) the reaction proceeded with lower yields. It seems reasonable to ascribe the positive role of 1 to the dispersion of the fluorophilic/lipophilic substrates in water as a result
6 Page 8 of 28
of the formation of micelles. It should be noted that direct evidence for the formation of
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micelles was not provided.
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Scheme 1: Structural formulas of fluorous surfactants 1-3, and reaction studied.[10]
Considering hydrosoluble substrates/reagents, generating micellar aqueous solutions
ed
can be beneficial for reactions employing hydrophobic catalysts. Kondo and coworkers, exploited surfactants to favor the reactivity of copper catalysts towards reactants solubilized
ce pt
in water.[11] Copper salts were combined with two types of ligand (scheme 2) and various surfactants (F-PEG, Brij-30 or Triton X-100), and tested the coupling reaction between arylboronic acids and imidazoles. CuOAc2-4 (10 mol%) was combined with either Triton X-
Ac
100 or Brij-30, while CuOAc-5 (10 mol%) was employed with F-PEG. They have shown that the yield of coupling product was approximately doubled when the surfactant is present (30 to 50 mol%). Exploiting the optimal conditions, a range of N-arylimidazoles was prepared in moderate to good yields, the choice of the best ligand/surfactant couple being substratedependent.
7 Page 9 of 28
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Scheme 2: Ligands, surfactants and reaction studied in the work of Kondo and coworkers.[11]
In the previous examples, the surfactants used were not the catalytic species. Hanzawa
an
et al. used the perfluooctanesulfonic acid (PFOSA) as a surfactant catalyst,[12] a so-called Brønsted-Acid-Surfactant-combined Catalyst.[13] Various acids were tested for the Pictet-
M
Spengler reaction (scheme 3). For the model reaction, the best yield (90% after 18 h) was
ed
obtained with PFOSA employed at 20 mol% loading. For comparison, using the trifluoromethylsulfonic acid (20 mol%) the yield was 4%. The authors suggested that the
ce pt
substrates and the PFOSA formed aggregates in water. The addition of hexafluoroisopropanol was found to improve the reactivity, with a yield reaching 99% in only 4 h. Using the optimized conditions, they succeeded in preparing a range of tetrahydroisoquinolines in water
Ac
with good to excellent yields.
Scheme 3: Pictet-Spengler reaction.[12]
8 Page 10 of 28
Similarly, Zhang et al. used the zinc perfluorooctanoate, Zn(PFO)2 as a Lewis Acid Surfactant Catalyst (LASC).[14] They characterized its surfactant properties by measuring its CMC (5 mM) and the minimum water/air surface tension (19.76 mN/m). It appeared that Zn(PFO)2 was a surfactant for water as well as for organic solvents. Zn(PFO)2 was applied as
ip t
catalyst (2.5 mol%) for the preparation of quinazolinones in water or water/ethanol mixtures. As shown in the figure 1, the solution micelle-containing solution turns from homogeneous
cr
(fig. 1, a) to an emulsion (fig. 1, b) as soon as the non-soluble reactants were introduced. At
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the end of the reaction, the products which precipitate upon cooling the reaction mixture were recovered by filtration, while the filtrate containing the micellar catalyst (fig.1, c) could be
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ed
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an
reused for subsequent reactions.
Figure 1: Photographs of: (a) Zn(PFO)2 in water; (b) emulsion solution after adding reagents;
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(c) the precipitated product at the end of the reaction (extracted from [14]).
1.2 In scCO2
The direct incorporation of fluorinated groups to ligands allows consideration of the
use of their metal complexes as surfactant catalysts at the interface of dispersed and continuous phases. When one phase consists of scCO2, it entails additional advantages: Firstly, scCO2 allow the replacement of harmful and expensive organic solvents, conducive with improved sustainability in chemical processes. Secondly, chemists have additional means to achieve more effective syntheses since the solvation properties of scCO2 can be 9 Page 11 of 28
finely tuned by adjusting the pressure and temperature. The catalytic hydrocarboxylation of terminal alkenes with CO and a surfactant-like catalyst were conducted at high pressure in inverse systems, i.e. liquid water-in-supercritical CO2 (scheme 4).[15] The catalyst is formed in situ from precatalyst [PdCl2(NCPh)2] and phosphine ligands. The latter could contain one
us
cr
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or several short fluorinated groups (-CF3).
system
was
the mixture of 1 mM
of palladium
precatalyst
with
M
The best
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Scheme 4: Pd-catalyzed hydrocarboxylation of terminal alkenes in supercritical conditions.[15]
tris(4-trifluoromethylphenyl)phosphine, oxalic acid, 1-octene and water in a molar ratio
ed
1:4:62.5:62.5:500, respectively, with the following optimal experimental conditions: 12 h at 90 °C and 3 MPa of CO for a total pressure of 15 MPa after CO2 addition. Interestingly, high
ce pt
conversion (55 %) and selectivity (90 %) were achieved when the precatalytic system was not soluble in the supercritical mixture of CO and CO2. At higher pressure, when the precatalyst becomes soluble in the supercritical medium, almost no conversion was observed. The best
Ac
results were obtained when a perfluorinated surfactant (Krytox®) was added to the system (93 % conversion and 77 % selectivity). Decreasing the amount of the surfactant leads to a lower conversion while selectivity is not affected. Although no physico-chemical study of the catalytic system was conducted, the improvement of the conversion could be ascribed to the ambivalent character of the surfactant catalyst and the addition of the perfluorinated cosurfactant. Indeed, the latter was used in order to improve the generation and stabilization of smaller water droplets into the supercritical medium, and thus to increase the interfacial area, i.e. where the catalyst could be confined thanks to the fluorinated moieties of its ligands. 10 Page 12 of 28
Moreover, one can note that venting the reactor containing the supercritical medium allowed extraction of the carboxylic acids produced in a cold trap. Such water/scCO2 medium was also used with success by Tsang et al. to perform the catalytic air oxidation of toluene.[16] The authors synthesized the surfactant catalyst
ip t
[CF3(CF2)8COO]2Co·nH2O and used it with NaBr promoter in a water-in-scCO2 system containing toluene and O2 (1 MPa) at a total pressure of 15 MPa and 120 °C (scheme 5). After
cr
an induction period corresponding to the wetting of the scCO2, their catalyst showed excellent
us
activity in these conditions: 98 % conversion, 99 % selectivity to benzoic acid and a high turnover frequency (TOF = 6.19 x 10-3 s-1), while poor activity was observed if NaBr or water
an
was not included in the chemical system. Interestingly, their unoptimized catalyst is about one or two orders of magnitude more reactive than regular catalytic systems run in acetic acid-
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water or employing solid Co in scCO2. They attributed such high activity to the intrinsically faster transfers occurring in supercritical fluids than in conventional liquids, the dynamic
ed
properties of the emulsion, and the local high concentration of chemicals in the emulsion droplets and at the interface of the disperse and continuous phases. However, they did not
ce pt
show whether this was a truly stable microemulsion or a coarsely dispersed biphasic system. It is interesting to note that at standard ambient temperature and pressure, i.e. 25 °C and 0.1 MPa, the catalytic system is a solid, and hence a solid foam was obtained after rapid
Ac
depressurization of the reactor. The authors also demonstrated that after a first complete air oxidation of toluene, the inverse catalytic system containing the surfactant-like catalyst can be reused for a second run with no sign of deactivation.[17] Moreover, the authors showed that by shortening the fluorinated tail of the anionic surfactant CF3(CF2)nCOO- a significant increase of the reaction rates was observed (reaction times of 9.7, 10.5 and 12.3 h for n = 3, 8 and 12, respectively). As a small change in the surfactant structure can modify the surface properties of the micelle, it significantly alters the reactivity of the substrates. In other words,
11 Page 13 of 28
the toluene which was confined in the supercritical phase could reach the catalytic head of the surfactant-like catalyst more easily when it bore a short fluorinated tail. Again, this work shows the advantage of designing and using a surfactant catalyst when reactants of different polarities are located in different solvents. Use of scCO2 enhances the heat and mass transfer
ip t
as well as improving the process by allowing an easier separation and reusability of the
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catalytic micellar system.
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Scheme 5. Schematic representation of the catalytic system in a water-in-scCO2 mixture (extracted from [17]).
The same group also reported the use of a surfactant catalyst based on the combination
Ac
of ammonium perfluorotetradecanoate and palladium nitrate (1:1 molar), applied to the hydrogenation of citral under wet scCO2 conditions (scheme 6).[18] A slight excess of water was added to the wet CO2 in order to obtain a single and transparent supercritical phase at 40 °C and 14 MPa containing micelles hosting Pd nanoparticles (NPs). The excess of water-tosurfactant molar ratio must be between 2 to 30 to obtain monodispersed Pd NPs of ~4 nm with a cubo-octahedral shape. To achieve the reaction, the reactor was then filled with 1 MPa of H2 (the total pressure was then increased up to 15 MPa) and the hydrogenation of citral was
12 Page 14 of 28
conducted during 1 h until completion. Surprisingly, an unusual selectivity was obtained in this inverse liquid water-scCO2 microemulsion, the citronellal being the preferred product (68 %) compared to dihydrocitronellal (12 %) respectively, as opposed to vapor (1 % and 82 %) or solution processes (19 % and 66 %). This result was explained by the authors by the size
ip t
and the shape of the micelles that consists in a water core containing the Pd nanoparticles surrounded by the extended long fluorinated chains of the surfactant. Hence, citral could align
cr
itself to allow its hydrophilic head to enter in the water core of the micelles. This behavior
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should allow the selective hydrogenation of the conjugated double bond that should be
ed
M
an
near the Pd surface.
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Scheme 6. Products formed upon hydrogenation of citral.
2. Reactions under biphasic conditions 2.1 In emulsions
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Generating an emulsion leads to an increase of the surface area between two immiscible phases. If the emulsion is stabilized by a fluorosurfactant catalyst, reactions could then be potentially conducted effectively under biphasic conditions, while keeping the reactants/substrates and the catalyst compartmentalized, thus facilitating the product isolation and catalyst recycling by simple decantation. Since the publication by Horváth and Rábai in 1994 of a seminal paper on the Fluorous Biphasic Catalysis (FBC) [19], the most commonly employed metal ligands have been modified with perfluoroalkyl tags, and their metal complexes prepared and characterized to be employed in catalytic processes [20]. Depending 13 Page 15 of 28
on the fluorophilicity of the complexes, convenient and very efficient catalysts recovery procedures were developed based on liquid/liquid or solid/liquid separation protocols. When considering the chemical structure of the fluorous complexes of transition metals, it appears that they are intrinsically amphiphilic compounds. They typically possess a hydrophobic or
ip t
even lipophobic domain due to the presence of at least one perfluoralkyl chain (typically –C8F17) in their structure, while the hydrophilic domain should be provided by the metal
cr
ion/counterion couple. Most of the fluorous metal complexes described up to now could
us
potentially behave as moderate to very strong surfactants, with the possibility of stabilizing, depending on their fluorophilicity/lipophilicity, not only HC/water interfaces, but also
an
PFC/water or PFC/HC interfaces. It is also expected that the rigidity of their perfluoroalkyl tags will favor their self-organization at interfaces into highly organized and densely-packed
M
structures. Surprisingly, to the best of our knowledge, despite the great number of fluorous transition-metal complexes developed so far and employed in multiphasic catalytic systems,
ed
no studies have been reported yet to try to evaluate their surfactant character, in particular by assessing their ability to lower surface tensions of liquid/liquid interfaces. Nonetheless, we
ce pt
wish to highlight below some examples of fluorous biphasic catalysis, in which catalysis occurs at liquid/liquid or air/liquid interfaces and for which the surfactant properties of the fluorous catalyst may be invoked to account for the observed reactivity.
Ac
Reiser et al. prepared the fluorous “click” TEMPO derivative 6 (scheme 7) and
showed that this compound promotes the formation of an emulsion in a DCM/water mixture [21]. It was found that its solubility was very low in most solvents including DCM, perfluoromethylcyclohexane (≤ 1 mg/10 mL) and water. Under heterogeneous DCM/water conditions and using NaOCl as oxidant, 6 (0.2 to 1 mol%) proved a highly reactive and selective catalyst for the oxidation of alcohols to aldehydes. Its reactivity was even slightly higher than that observed with the pristine TEMPO which was fully soluble in the reaction
14 Page 16 of 28
conditions and did not emulsify the biphasic reaction mixture. The authors suggested that the high reactivity of 6 could be attributed to the emulsification of the reaction mixture by the catalyst, leading to a surfactant effect. Most interestingly, the recycling of 6 could be achieved by simple filtration followed by washings, while no loss of activity was noticed in four runs
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using an initial amount of 1 mol% of 6.
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6
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Scheme 7: Structure of the fluorous TEMPO 6 (Rf8 = C8F17) and photograph of the emulsified biphasic reaction mixture containing 6 (0.2 mol%) and the substrate/reactants (photograph
ed
extracted from [21]).
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Nishikido and coworkers reported on a practical fluorous biphasic process for small and bench-scale esterifications catalyzed by the lanthanide(III) complex Yb[N(SO2-nC8F17)2]3.[22] They developed a continuous-flow apparatus consisting of a reactor with a
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mechanical stirrer and a separate decanter where phase-separation occurs (scheme 8). Reactions were conducted at 40 °C under biphasic conditions, the fluorous catalyst being immobilized in a PFC, while the substrate (alcohol)/reactant (acetic anhydride) are dissolved in toluene. The authors highlighted the fact that the reaction were conducted under vigorous stirring and proceeded in the resultant emulsion. Using a semi-industrial set up (reactor of 500 mL) the acetylation of cyclohexanol by acetic anhydride proceeded successfully for more than 500 h with high TON (~10000) attained for the catalyst whilst very low leaching of Yb was detected. This catalytic system also proved to be highly efficient for the Baeyer-Villiger 15 Page 17 of 28
reaction. Overall, this work clearly demonstrated the potential of such continuousflow/fluorous biphase systems for industrial applications.
during reaction
products in organic solvent
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before reaction
emulsion
organic
substrates in organic solvent
fluorous
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reactor
Phase separation
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decanter
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catalyst immobilised in fluorous phase
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Scheme 8: Continuous-flow reaction model based on FBC (adapted from [22]).
Other interesting examples of catalytic reactions conducted using emulsified PFC/HC
ed
biphasic systems were reported by Gladysz and co-workers [23] and by Friesen, Montgomery and co-workers[24]. They prepared fluorous Wilkinson-type catalysts 7 [23] and 8,[24] the
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main difference being that 8 incorporated perfluoropolyalkylether chains instead of typical – C8F17 ponytails (scheme 9). Perfluoropolyalkylether chains are of particular interest as they raise less environmental concerns than the –CnF2n+1 chains and provide increased
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fluorophilicity.[25] The hydrogenation of 2-cyclohexen-1-one catalysed by 7 [23] or 8 [24] was tested in the conditions of initially employed by Gladysz and co-workers,[24] i.e. under 1 atm of H2 at 45 °C in a biphasic perfluoromethylcyclohexane (PFMCH):toluene mixture. The reactions were conducted under vigorous stirring, the term emulsion being employed by Friesen and Montgomery to describe the reaction mixture. Both catalysts exhibited excellent activities with TOF values of 31 h-1 and 47 h-1 for 8 and 7, respectively. Interestingly, it was noticed that the reactions proceeded more effectively under the “emulsified biphasic conditions” than under homogeneous monophasic conditions,[23] for instance employing a 16 Page 18 of 28
1:3:3 solution of toluene:hexanes:PFMCH which forms a single phase at 45 °C. Whether this increased reactivity could be attributed to a surfactant effect brought or amplified by the fluorous catalyst is possible, but further studies would be necessary to assess this interesting
8
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7
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point.
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Scheme 9: Structures of the fluorous Wilkinson’s-type catalysts 7 [23] and 8.[24]
Yi and Cai conducted organic reactions at a water-fluorous solvent interface. They
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showed that the synthesis of benzodiazepines (scheme 10), through the condensation of ophenylenediamine and various ketones, could be conducted very effectively at room
ed
temperature when running the reactions in an aqueous/fluorous emulsion, the best results
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being obtained for the water/perfluorooctane/KFOS (potassium perfluorooctanesulfonate) system.[26] When using water, an organic solvent, or a PFC only as reaction medium, the reaction did not proceed, while in a water/PFC mixture without surfactant the reaction yield was very low. They proposed that the rate enhancement was due to the confinement of the
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reactants at the water/PFC interface generated in the stable emulsion which formed upon stirring, the average particle size diameter being ~ 3 microns.
Scheme 10. Synthesis of benzodiazepines.[26]
17 Page 19 of 28
2.2 In microfluidic devices During the last decade, droplet-based milli/microfluidic devices have received considerable interest.[27] As far as chemical reactions are concerned, reactions are typically conducted within individual microdroplets in an immiscible continuous phase which carry
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these individual microreactors in microfluidic channels. PFCs have emerged as particularly appealing fluids for the continuous phase as they can carry either water-based or organic
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solvent-based microdroplets. In 2009, Theberge, Huck and co-workers exploited a fluorous
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Pd(II) complex as a PFC-soluble catalyst with amphiphilic properties which could accumulate at the PFC-water interface and thus favor the reactivity at the interface between the reactants
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solubilised in the water droplets and the catalyst dissolved in the continuous fluorous phase (scheme 11).[28] Using the setup presented in scheme 11, Suzuki couplings have been
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conducted from a range aryl bromides and aryl boronic acids affording the corresponding biaryls in good to excellent yields using residence times ranging from 0.75 to 8 h. Continuous
ed
recycling of the catalyst was successfully achieved by recirculating the fluorous phase, the catalyst being still active after having being recirculated approximately four times. The
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catalyst leaching from the fluorous phase was found very low, i.e. less than 2% of the total palladium. Interestingly, despite the precipitation of most of the biaryls forming within the
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aqueous droplets, the plugging of the PTFE tube was not observed.
18 Page 20 of 28
a d
c
e
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b
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Scheme 11: a) Structure of the fluorous Pd catalyst; b) schematic representation of a water droplet carried in a channel by the fluorous phase containing the catalyst; c) Schematic
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representation of the microfluidic set-up; d) T-junction for the droplet formation; e)
3. Reaction at the air-water interface
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Separating column. Adapted from [28].
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An important contribution was made by Mingotaud and co-workers who exploited Langmuir films as a tool to study how the orientation/confinement of a catalyst, as found in
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heterogeneous or densely packed catalytic systems, may influence its reactivity.[29] Indeed, Langmuir films represent a unique opportunity to finely control and tune the density and orientation of a catalyst at the air-water interface, and thus study the influence of such
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parameters on the catalytic activity. They chose the fluorous Mn-salen complex 9 as the amphiphilic catalyst for the epoxidation of the cinnamyl alcohol using the Urea/Hydrogen Peroxide (UHP) as the oxygen source (scheme 12).
19 Page 21 of 28
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Scheme 12: Structure of the amphiphilic catalyst 9 and reaction studied by Mingotaud and co-
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workers.[29)]
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Complex 9 forms a homogeneous and stable Langmuir film at the air/water interface, the molecular area at collapse being found at around 115 Å2. Interestingly, when the UHP was
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present in the aqueous phase, the monolayer was found to be denser at a given pressure, thus suggesting a strong interaction with 9. Catalytic experiments were conducted by spreading 9
ed
on the aqueous phase containing the alcohol and UHP and determining the % of epoxide
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formed after a given time for different compression states of the film (figure 2). These data show that when compressing the film until a molecular area of ~ 150 Å2 (surface pressure kept at 0), a linear-dependency of the apparent rate with the molecular density is observed, as expected as no molecular organisation is present in the liquid phase of the monolayer. This
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also agrees with observations in bulk solutions for which the reaction was found to be firstorder with respect to the catalyst. When compressing the film further (area < 150 Å2), a strong enhancement of the epoxide formation was observed, the maximum being attained for an area per molecule of about 140 Å2. It is proposed that the increased reactivity is due to the modification of the catalyst orientation during compression, PM-IRRAS measurements revealing an increase in the monolayer thickness during compression, as well as different orientations of 9 at low and high surface pressures. At very high surface pressures (molecular
20 Page 22 of 28
area below 135 Å2), a sharp decrease of the recovered epoxide was observed, most probably explained by the slow collapse of the film. By exploiting the “natural” surfactant properties of a fluorous complex and its ability to self-assemble into a highly organised langmuir film at the air/water interface it is thus possible to get important information on the influence of the
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organisation of a catalyst in a surface-confined environment. There is little doubt that this approach exploiting fluorous catalysts could be extended to the study of enantioselective
cr
catalytic processes, or to systems which form Langmuir films at the air/organic solvent
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ed
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interface.
Figure 2: Molar percentage of epoxide recovered from the subphase after 6 h of reaction for
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various areas per molecule of 9 (extracted from [29b]).
Conclusion
While fluorosurfactants are used in a broad range of applications, their use as additives in catalytic processes has received little attention. The examples highlighted in the review show the potential of fluorosurfactants in favoring reactions that take place at the interface between two phases, in particular if one of the phases is a PFC or scCO2. We expect that
21 Page 23 of 28
studies in this field will be pursued. During the last 20 years, with the development of the fluorous chemistry, almost all catalysts (metal-based or organocatalysts) have been developed in a fluorous version. Such compounds, which integrate at least one perfluoroalkyl chain in their structure should exhibit significant surfactant properties. As shown by the works of
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Reiser or Theberge, such fluorous catalysts have a great potential for reactions conducted in liquid/liquid processes, by stabilizing emulsions and/or accumulating at the interface of
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PFC/water or PFC/HC biphasic systems. However, more studies need to be conducted to
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assess the surfactant properties of the fluorous catalysts, and thereby allowing evaluation of the extent to which the surfactant character of the catalyst is crucial for the observed activity.
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When conducting reactions under homogeneous micellar conditions with conventional surfactants, the separation of the surfactant from the products could be problematic. If using
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fluorosurfactants, Fluorous Solid-Phase Extraction (FSPE) using fluorous silica-gel as the stationary phase, may represent a powerful separation technique to recover/recycle the
ed
surfactant.[30] Finally, the environmental concerns raised by the accumulation of the perfluorinated compounds in the environment and, for some of them, in living organisms
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(PFOA, PFOS…), could represent a strong limitation for further developments in this area.[31] However, one should note that for applications in catalytic processes, the confinement of the reactants coupled to effective recovery techniques, will obviously lead to
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minimal leaching into the environment. We thus believe that environmental issues should not hamper innovative researches for applications in catalysis. However, further studies in the field should favor the use of fluorosurfactants with less environmental/health impact, for instance those bearing perfluoroalkylether or shorter perfluoroalkylated (≤ -C6F13) fragments.
22 Page 24 of 28
Acknowledgments The authors warmly thank the CNRS and Solvay for funding this work. The University of Bordeaux, the Région Aquitaine as well as Solvay RIC Bordeaux are also acknowledged for
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their support.
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